In previous studies, we examined hybrid viral RNAs and proteins synthesized in both human and monkey cells infected by non-defective adenovirus SV40 hybrid viruses. A detailed analysis of the hybrid viral transcripts synthesized from AD2+ND4 stocks was performed. These stocks were shown to be nonhomogeneous, consisting of two principle populations of viral DNAs. Both populations contained segments of SV40 DNA extending from map position 0.63 to 0.11 inserted at adenovirus map position 0.79 in a left to right orientation. Thus, they can be transcribed either from the early or late adenovirus promoters. One population, however, contains an intact SV40 DNA segment (about 15% of the molecules), while the major population (85%) contains an SV40 deletion extending from .60 to .50 SV40 map units. This deletion encompasses the SV40 DNA segment encoding early SV40 splice sites; we presume that growth of the viral stock in human cells is inhibited by the presence of these splice junctions. Three major hybrid viral transcripts synthesized late in a lytic infection by these viruses were analyzed in detail. The least abundant species is transcribed from the